1. Field of the Invention
The present invention relates generally to satellite communication, and more particularly to a method for acquiring a signal of a satellite.
2. Description of the Related Art
When a Global Positioning System (GPS) receiver is activated after being in the off position for a long time, it can take several minutes before the time to first fix is reached. There are a number of approaches which aim to reduce the time to first fix by means of different algorithms. Most current approaches rely on a priori data that should be obtained in advance, which causes the receiver to lose its independence. Examples of such methods are described in the following patents: U.S. Pat. No. 5,663,735 to Eshenbach, entitled “GPS receiver using a radio signal for improving time to first fix”, and U.S. Pat. No. 5,854,605 to Gildea, entitled “GPS receiver having a fast time to first fix”.
In these patents the time to first fix is reduced due to availability of time data obtained externally or by use of the receiver clock.
Other known methods calculate rough current satellite position using the previously known almanac or previous ephemeris values. An example of this approach is described in U.S. Pat. No. 6,275,185 to Loomis, entitled “GPS receiver using coarse orbital parameters for achieving a fast time to first fix”.
Some of the prior art methods use the information about the approximate satellite position as well as other priori data, which quickens the system start time. Such methods are described in U.S. Pat. No. 7,215,967 to Kransmo et al, entitled “System and method for fast cold start of a GPS receiver in a telecommunications environment”, and in U.S. Pat. No. 6,813,500 to Cinager et al, entitled “Cellular telephone using pseudolites for determining location”.
U.S. Patent Publication No. 2007/0229351 to Chen et al. (hereinafter '351) discloses a cold start satellite search method. FIG. 1 illustrates a flowchart of the cold start satellite search method according to '351. Referring to FIG. 1, in step 101, a set of L satellites and weighting factors previously calculated for the satellites are pre-stored in a memory of a receiver. In step 111, a request signal for satellite signal acquisition is received, and the data stored in the receiver is initialized in step 112. That is, a determination on satellite searchability is initialized, an initial value is allocated to the weighting factors (Wk), and a candidate set for a satellite search is formed. In step 113, pursuant to a predefined rule, a satellite with the lowest number allocated thereto is selected from among the satellites set in the candidate set.
In step 114, a search is performed for a signal of the selected satellite. If a signal of the selected satellite is detected in step 115, then a determination is made in step 116 that the corresponding satellite has a searchable satellite signal, the weighting factor of the corresponding satellite is updated in step 117, and the corresponding satellite is removed from the candidate set in step 118. In step 119, it is determined whether a first satellite has been acquired The satellite search ends when the first satellite has been searched for, and the method proceeds to step 124 when the first satellite is not searched for. Contrarily, if a signal of the selected satellite is not detected in step 115, then in step 121 a determination that the corresponding satellite has an unsearchable satellite signal is made, the weighting factor of the corresponding satellite is updated in step 122, and the corresponding satellite is removed from the candidate set in step 123.
In step 124, it is determined whether there is any other satellite included in the candidate set. When no satellite remains in the candidate set, satellites having signals that are not searched for are reset to the candidate set in step 125. When any other satellite remains in the candidate set, in step 126 a satellite with the highest weighting factor is selected from among the satellites included in the candidate set in consideration of the weighting factors for the respective satellites, and then the method returns to step 114.
The above-described '351 publication cannot provide a fast Time-To-First-Fix (TTFF) under the conditions of a priori uncertainty. More specially, in the actual environment, a satellite may provide a signal to a receiver, but the receiver may not detect the signal provided from the satellite. Due to such an error in satellite signal detection, a weighting factor may be erroneous, which results in a problem of delaying a satellite signal search time. Also, there is another problem in that it may take a relatively long time to complete a satellite search because satellites having signals that are not searched for may be continuously returned to the candidate set. Further, the '351 publication has a problem in that it cannot perform searches actively against dynamic changes caused by errors of a satellite or an acquired satellite's movement below the horizon.